1. Field of the Invention
This invention relates to a coordinate measuring machine, especially to an improvement of a drive mechanism for a gate-type slider of the coordinate measuring machine.
2. Description of the Related Art
It is known that various kinds of coordinate measuring machines are used for accurate measurements of the dimensions and configuration of an object to be measured.
One of the typical machines has a gate-type slider which moves on a table thereof. Since the slider can go aside on the table, the object is easily placed onto and taken off from the table and the capacity of measurement is high, so that this type of machine is commonly used.
One example of this machine is illustrated in FIGS. 4 and 5.
The typical coordinate measuring machine 60 has a table 61 supported on the floor or ground. A slider device 70 is supported on the table 61 and on which an object W to be measured is to be placed.
The slider device 70 is fit together with a Y-slider 71 shaped like a gate and moving in a Y-direction, a X-slider 72 moving in a X-direction along the Y-slider, and a Z-slider 74 having a measuring element 73 and moving in a Z-direction through the X-slider 72.
The Y-slider 71 is constructed with two columns 71A, 71B and a beam 71C which crosses from one to another of the columns 71A, 71B so as to have a shape of the gate. At a side 61A of the table 61, a driving device 80 for moving the Y-slider 71 in the Y-direction and a guide member 85 are provided for the column 71B. The driving device 80 consists of a motor 81 and a ball screw 82 connected with a shaft of the motor 81 and being rotated in direction of the arrow R.
The X-slider 72 and Z-slider 74 are also provided with respective driving devices not shown so as to move in individual directions X, Z.
The measurement of the object W by the coordinate measuring machine 60 is done as follows. First of all, the motor 81 is operated to make the ball screw 82 turn along the R-direction, which causes the Y-slider 71 to move in the Y-direction. Subsequently, the driving devices of the X-slider 72 and Z-slider 74 are so operated that the measuring element 73 comes into contact with the object W to obtain the dimensions or traces the outer surface of the object W to obtain the shape.
It is known well that a center of gravity for the slider 70 in the coordinate measuring machine 60 is around a point denoted by "G" in FIGS. 4, 5.
A moment of inertia I of the center of gravity G can be defined by a formula I=.alpha.ML; wherein M is a weight of the slider 70, .alpha. is an acceleration of the slider 70 moving toward Y-direction and L is a distance from the center of gravity G to the driving device 80. Incidentally, the distance L is rather long, so that the moment of inertia I becomes big in proportion thereto.
Hence, such big inertia I causes the Y-slider 71 to tilt, swing and vibrate, whenever the Y-slider is moving, so that the accuracy of measurement for the configuration of the object is lost.
It is natural to decrease the acceleration .alpha. of the Y-slider 71 in order to minimize the inertia I, but this is not preferable for a fast measurement. Otherwise, the inertia I may be minimized by decreasing the weight of the slider 70, but this is not preferable to keep enough contractual strength.
It is then considered to make the distance L short by placing the driving device 80 near the center of gravity. But, it is necessary for the driving device 80, having the motor 81, bearing and the like, to be provided at a special support throughout the Y-direction of the table 61. The special support for the driving device 80 is an obstacle for the free placement of the object W onto and off from the table 61, which is a demerit of the conventional gate-type coordinate measuring machine in view of price and space.
One of the objects in the present invention is to provide a coordinate measuring machine which is not affected by an inertia and performs fine measurement, while keeping some merits of the conventional coordinate measuring machine.